In recent years, particulates (fine particles) contained in exhaust gases discharged from internal combustion engines of vehicles such as buses, trucks and the like and construction machines have raised serious problems as these particles are harmful to the environment and the human body.
There have been proposed various ceramic filters which allow exhaust gases to pass through porous ceramics and collect particulates in the exhaust gases to purify the exhaust gases.
Conventionally, in the ceramic filter of this type, a number of through holes are placed in parallel with one another in one direction and wall portion that separates the through holes from each other functions as filters.
In other words, each of the through holes formed in the ceramic filter is sealed with a plug at either of ends of its exhaust gas inlet side or outlet side so as to form a so-called checkered pattern; thus, exhaust gases that have entered one through hole are discharged from another through hole after having always passed through partition wall that separates the through holes from each other. Consequently, when the exhaust gases pass through the partition wall, the particulates are captured by the portion of the partition wall to be purified.
As such a purifying process for exhaust gases progresses, particulates are gradually accumulated on the partition wall that separates the through holes of the honeycomb filter from each other to cause clogging and the subsequent interruption in gas permeability.
In order to solve this problem, there has been developed a honeycomb filter of a back-washing system, which, after having collected particulates, forms a gas flow in a direction reversed to the flow-in direction of exhaust gases so as to remove the particulates; however, this system requires a complex structure, and fails to provide a practical system (see JP Kokai Hei 7-332064).
For this reason, the above-mentioned honeycomb filter needs to be regularly subjected to a recycling process in which the particulates that cause clogging are burned and removed by using heating means such as a heater or the like to regenerate the filter.
Here, in the conventional honeycomb filter having the above-mentioned structure, the region capable of purifying the exhaust gases (hereinafter, referred to as a filtration capable region) corresponds to the inner wall of the through hole that is opened on the exhaust gas flow-in side. In order to maintain the filtration capable region as wide as possible in the honeycomb filter and also to keep the back pressure upon collection of particulates at a low level, it is profitable to make the length of a plug in the length direction of the through hole as short as possible.
Moreover, in the case where the porosity of the honeycomb filter is low, the back pressure becomes higher quickly upon collecting the particulates, with the result that the above-mentioned recycling process using the heating means such as a heater or the like needs to be carried out frequently; therefore, an attempt to make the porosity of the honeycomb filter higher has been made conventionally.
In recent years, another technique has been proposed in which, in place of the above-mentioned recycling process of the honeycomb filter using the heating means such as a heater or the like, by allowing the honeycomb filter to support an oxidizing catalyst in its pores, hydrocarbon contained in exhaust gases that flow into the honeycomb filter is made to react with the oxidizing catalyst, then heat generated through this reaction is utilized for the recycling process. In the honeycomb filter that carries out the recycling process in this manner, it is necessary to increase the porosity thereof, because the oxidizing catalyst is supported on the inside of each pore of the honeycomb filter so that the pore becomes more likely to cause clogging due to particulates, and because the oxidizing catalyst needs to be supported as much as possible in order to generate a large amount of heat, or other reasons.
By increasing the porosity of the honeycomb filter in this manner, it becomes possible to prevent the back pressure from becoming higher, to provide a superior particulate collecting property, and also to allow the filter to support a large amount of oxidizing catalyst.
However, the increase in the porosity of the honeycomb filter causes a reduction in the strength of the honeycomb filter itself. For this reason, when an exhaust gas purifying apparatus, to which the honeycomb filter is attached, is installed in an exhaust gas passage of an internal combustion engine such as an engine or the like, and actually used, cracks tend to occur in the partition wall due to an impact caused by a pressure and the like from the exhaust gases.
Moreover, as described above, the plug to be injected into the end of the through hole is formed to have the length in the length direction of the through hole, which is set as short as possible, in order to maintain the filtering capable region as wide as possible; however, the honeycomb filter of this type has a small contact area between the plug and the partition wall, resulting in a reduction in the adhesion strength of the plug to the partition wall (see JP Kokai 2003-3823).
Here, the portion of the partition wall in which the plug is injected on the outlet side of exhaust gases corresponds to a portion that is to have a highest impact from the pressure and the like from the exhaust gases; consequently, in the case of the honeycomb filter having a reduced bending strength due to the above-mentioned increased porosity, the partition wall in which the plug is injected is more likely to cause: occurrence of cracks due to an impact caused by a pressure and the like from the exhaust gases; and the subsequent coming-off of the plug, resulting in degradation in the durability.